1. Technical Field
The present invention relates to a parking brake lock-in key switch system for a truck or bus air brake system.
2. Description of the Problem
In prior art air brake systems used on truck and bus chassis manufactured by International Truck and Engine Corp., separate air lines run from the primary air tank and the secondary air tank to a parking brake actuator. The parking brake actuator is a push pull double check (PPDC) valve, which aligns the tank with the higher pressure to supply air to the brake chambers for individual wheel parking brakes. When pulled out to an ‘Engaged’ position, the push pull portion of the actuator closes off the air supply passageway from the air tanks and vents a single downstream air line. The single downstream air line from the PPDC valve runs from the valve through an inversion (relay) valve and a quick release valve to spring brake chambers which act in a parking brake mode. The spring or park brake chambers are part of the individual brake assemblies for each wheel subject to park braking. Typically, the vehicle's rear wheels have the brake assemblies which act as parking brakes in addition to having a service brake function. The springs in the spring brake chambers act to lock the individual wheels when there is no air applied to counter the spring force. Operation of the push pull knob of the parking brake actuator with its resulting venting of the down stream air piping to the brake chambers allows the spring brakes to lock the individual wheels associated with the park braking. In normal operation the spring brake chambers lock the individual wheels only so long as the parking brake actuator is in the ‘Engaged’ or pulled out position.
Vehicles with hydraulically operated service brakes have had air assisted drive line parking brakes in the prior art. While the hydraulic service brakes act directly on the wheels of the vehicle, the drive line parking brakes act to lock the drive line prop shaft of the vehicle. An air brake cylinder is engaged to the drum in the driveline which locks the shaft. The air brake cylinder allows the shaft to rotate when air is supplied to the cylinder. The air is provided from a single air reservoir through a parking spring brake control through a one-way check valve to the air brake cylinder. When the driver operates the parking spring brake control to the engaged position, the air supply to the cylinder is stopped and the downstream piping is vented. Upon the venting of the piping between the spring brake central and the air brake cylinder, the air brake cylinder will cause the drum brake to lock the driveline prop shaft. Similar to the above described air brake system, the drum brake will only continue to lock the prop shaft so long as the parking spring brake control is in the ‘Engaged’ position.
Automatic parking brake systems exist in the prior art which automatically vent downstream air lines supplying brake chambers or cylinders associated with park braking upon the operator turning the ignition key to an ‘off’ position. Automatic engagement of the parking brakes follows. A problem with such systems is that a driver or passenger may inadvertently engage the parking brakes while the vehicle is operating at highway speeds by bumping the ignition key to a non-operating or ‘off’ position. Involuntary braking could result. In addition, bus engines are not typically shut off at bus stops, with the result that the system does not operate when needed.
U.S. Pat. No. 6,234,586 describes vehicles such as school buses equipped with air brake systems and having a driver operated parking brake actuator. Following manual engagement of the parking brake actuator, the parking brake lock-in key switch system will lock the parking brake in the engaged position upon the driver turning the ignition key to the ‘off’ position. This effectively disables the parking brake actuator thereby preventing inadvertent release of the parking brake. This system does not allow inadvertent initiation of the parking brake should the ignition key be moved to the ‘off’ position while the vehicle is in motion. The driver must first consciously operate the parking brake actuator for the parking brake lock-in key switch system. In some vehicles with automatic transmissions an automatic apply-automatic release parking brake system operates in parallel with the parking brake lock-in key switch system. The automatic apply-automatic release parking brake system has a valve that acts to operate and engage the parking brake when the automatic transmission is placed in the _Park_ position. The valve of the automatic apply-automatic release parking brake system in these systems deactivates and releases the parking brake when the transmission shifter is moved out of the _Park_ position and the engine of the vehicle is running. With automatic application of the parking brakes when the vehicle transmission is placed in park, but a requirement for service brake application, key on and deliberate brake release prior to release of the parking brakes, there is little chance of a bus passenger releasing the brake, even if the driver is not in his seat.
Air brake vehicles are available with a key and service brake interlock function which requires a driver to turn the ignition key to the ON position and to apply the service brake prior to releasing the park brake. It is desirable to provide a system which automatically applies the park brakes when the vehicle automatic transmission is shifted into the Park position. It is also desirable that the driver would then have to reapply the service brake prior to disengaging the parking brakes.